class_docstr: Test upgrade_opt behavior.
code: def test_train_fixed(self):
      args = self._get_args()
      args["candidates"] = "fixed"
      args["eval_candidates"] = "fixed"
      args["encode_candidate_vecs"] = False
      valid, test = testing_utils.train_model(args)
      threshold = self._get_threshold()
  
      self.assertGreaterEqual(valid["hits@1"], threshold)
  
imports: import os
  import pytest
  import unittest
  import parlai.utils.testing
  from parlai.tasks.integration_tests.agents import CandidateTeacher
label_desc: Use image node_id 6 for calling the _AbstractTRATest obj's underlying member method code with example usage: obj.test_train_fixed() without return types